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Lithium-thionyl chloride cells are widely used in downhole applications where the temperatures exceed 100°C. These cells cannot be used above the melting point of lithium, 180°C, but modified oxyhalide cells are available that use higher-melting lithium alloy anodes that allow safe operation at temperatures as high as 200°C. However, the higher temperature capability comes at the cost of low temperature performance; the alloy cells typically show very poor rate capability below 50°C. The low temperature rate limitations can be particularly disadvantageous in cases where a tool is started up at the surface, where the ambient temperatures are cooler, before it is placed into operation downhole. Here we present test results defining and characterizing the capabilities and limitations of various types of lithium alloy cells at lower temperatures, as well as discharge results at higher temperatures for new cell types that have been designed for improved rate capability at both lower and higher temperatures.

In lithium / thionyl chloride (LflC) cells, a lithium limited design was thought to be safer than a cathode limited design because the amount of lithium left in discharged cells would be minimal. However, lithium corrosion reduces the capacity faster than does cathode degradation during storage. The optimization of the ratio of lithium to c h o n was studied, considering storage time and temperature. The efficiency of converting chemical energy into electrical energy has been studied for the case of D cells with surface area from 45 to 345 cm2, under constant and various pulsed loads. Microcalorimetric monitoring of the heat output during discharge allowed the direct measurement of the faradaic efficiency, and showed that self-discharge is far more pervasive than previously acknowledged.' Typical faradaic efficiencies for constant load varied from 30% at low current density to 90% at moderate and 75% at high current density. Pulsed current further depresses these efficiencies, except at very low average current density.